Figure F24. Thin section photomicrographs of a zoned chert nodule from the Aptian section (Sample 198-1207B-37R-CC, 25-27 cm). This cross-sectional view shows that silicification was localized in a burrow filling where radiolarians (and porosity) were concentrated prior to alteration. The dark zone intervening between chert on the left and chalk on the far right consists of porcellanite with higher concentrations of opaque minerals, essentially a reaction front. The upper view is under plane-polarized light, whereas the lower view is with nicols crossed.

Figure F24. Thin section photomicrographs of a zoned chert nodule from the Aptian section (Sample 198-1207B-37R-CC, 25-27 cm). This cross-sectional view shows that silicification was localized in a burrow filling where radiolarians (and porosity) were concentrated prior to alteration. The dark zone intervening between chert on the left and chalk on the far right consists of porcellanite with higher concentrations of opaque minerals, essentially a reaction front. The upper view is under plane-polarized light, whereas the lower view is with nicols crossed.